1. Field of the Invention
The present invention relates to a loudspeaker unit in which a voice coil is energized to vibrate a diaphragm, and in particular, relates to a loudspeaker unit suitable to be installed in a vehicle cabin subject to severe space constraints or a space outside the vehicle cabin, e.g., an engine compartment.
2. Description of the Related Art
FIG. 9 is a cross-sectional view of a typical electrodynamic loudspeaker unit disclosed in, for example, Japanese Unexamined Patent Application Publication No. 62-272699. As illustrated in FIG. 9, a loudspeaker unit 100 includes a magnetic circuit 101 having a magnetic gap G, a voice coil 102 disposed in the magnetic gap G, the voice coil 102 being driven by electromagnetic interaction when energized, a substantially conical diaphragm 103 vibrating in unison with the voice coil 102, an annular damper 104 elastically supporting the voice coil 102 and the diaphragm 103, and a substantially conical frame 105 holding an outer edge of the diaphragm 103 and an outer edge of the damper 104.
The magnetic circuit 101 includes a bottom plate 106 having a center pole 106a, a ring-shaped magnet 107 placed on the bottom plate 106, and a top plate 108 disposed on the magnet 107. The magnetic gap G is defined between an outer circumferential surface of the center pole 106a and an inner circumferential surface of the top plate 108. The voice coil 102 is wound around a cylindrical bobbin 109. An inner edge of the diaphragm 103 and an inner edge of the damper 104 are bonded and fixed to upper end part of the bobbin 109. The frame 105 is fixed to an upper surface of the top plate 108. The frame 105 has a plurality of openings 105a arranged at regular intervals in a circumferential direction. The outer edge of the diaphragm 103 is held by an upper end of the frame 105, with an edge member 110 therebetween. A cap 111 is attached to central part of the diaphragm 103. The outer edge of the damper 104 is held by substantially intermediate part of the frame 105 in a vertical direction in FIG. 9.
A pair of lead wires 112, called tinsel wires, are connected to the voice coil 102. Audio current is supplied through the lead wires 112 to the voice coil 102. The lead wires 112 are routed out of a space between the diaphragm 103 and the damper 104. The tips of the lead wires 112 are connected to link terminals 113 attached to the frame 105. The lead wires 112 have to be in non-contact with the diaphragm 103 and the damper 104 during vibration of the voice coil 102. Such a non-contact state is provided by inserting a jig (not illustrated) into any of the openings 105a, forming free end portions of the lead wires 112 into a given shape (curved shape) using the jig, welding the tips of the lead wires 112 to the link terminals 113, and then removing the jig.
Another related-art loudspeaker unit with such a configuration further includes a cylindrical duct disposed so as to project from an outer surface of a conical frame which holds an outer edge of a diaphragm, the duct being allowed to communicate with an opening in a vehicle cabin wall (refer to International Publication No. WO 2011/047435, for example). In this related-art loudspeaker unit, a loudspeaker interior space defined by the diaphragm, a damper, and the frame is adjacent to an open end of the duct. Accordingly, air moved by vibration of the diaphragm flows into and out of the interior space through the duct, so that back pressure can be controlled using the additional mass of the duct (air resistance in the duct).
In the related-art loudspeaker unit disclosed in International Publication No. WO 2011/047435, the back pressure can be controlled such that sound pressure in a low frequency range is increased by appropriately setting the length and the diameter of the duct projecting from the outer surface of the frame. If this loudspeaker unit is a relatively large loudspeaker unit, such as a subwoofer, therefore, it can be installed in a vehicle cabin that is subject to severe space constraints. Since the duct projects from the outer surface of the frame which serves as an enclosure, a rear surface of the diaphragm is fully covered with the frame provided with the duct. Unfortunately, it is difficult to provide a sufficient space to form lead wires extending from a voice coil into a desired shape. In this case, the use of a damper provided with lead wires (tinsel wires) can eliminate forming the lead wires. However, such a damper is very expensive, leading to cost problems. Additionally, in terms of reliability, it is difficult to allow a loudspeaker unit with high drive power, e.g., a subwoofer, to include this damper.